1. Field of the Invention
The present invention relates to a method for manufacturing continuously cast slab for use in the production of grain-oriented electrical steel sheet and strip, more particularly, to a method for manufacturing continuously cast slab for use in the production of grain-oriented steel sheet or strip containing not more than 4.0% of Si, 0.01-0.08% of Al and 0.01-0.06% of S, wherein the Al content or the Al content and the S content of the continuously cast slab does not vary in the direction of the continuous casting. As a consequence, in accordance with the method of the present invention, there is obtained a continuously cast slab which is substantially uniform in Al or Al and S content throughout its length. When this slab is used to produce grain-oriented electrical steel sheet or strip, the product obtained is free from any variation or degradation of its magnetic characteristics in the longitudinal direction as a result of variation in the Al or Al and S content thereof.
2. Description of the Prior Art
As is well known, the continuous casting method has numerous advantages over the conventional ingot casting method and, therefore, has been adopted for manufacturing slab for use in the production of electrical steel sheet and strip.
A problem arises, however, when grain-oriented electrical steel sheet and strip containing Al or Al and S are produced from continuously cast slab, as will be clear from the following.
When the slab is manufactured by the continuous casting method, the last of the molten metal used in the casting operation does not leave the ladle (or other vessel) in which it is contained until the casting is completed, and this may be for as long as one hundred minutes or more. In other words, a part of the molten steel used for casting remains in the ladle throughout the considerably long period of time during which the continuous casting process is carried out. When the molten steel containing Al or Al and S is retained in the ladle for a long period of time, the Al and the S in the molten steel react with the slag covering its surface, and as a result, Al and S are transferred into the slag as Al oxide and Ca sulphide and these cause local variation in the Al or Al and S content of the molten metal. Thus, when this molten metal having local variations in Al or Al and S content is cast into a slab, the slab obtained varies in Al or Al and S content in the direction of casting. If this slab is used to produce grain-oriented electrical steel sheet or strip, the product obtained will be highly unstable in its magnetic properties in the longitudinal direction. It has, therefore, been impossible to use continuously cast slab for industrially producing grain-oriented electrical steel sheet and strip of the type containing Al or Al and S having uniform magnetic characteristics in the longitudinal direction.